1. Field of the Invention
The present invention relates to a two-cycle engine including exhaust control valves for varying open areas of exhaust ports. More particularly, to a structure for mounting the exhaust control valves near the exhaust ports. In addition, the present invention relates to a small boat which is driven by a rider sitting astride a saddle seat, and more particularly to a small boat including a power unit neatly housed in a limited space thereof
2. Description of Background Art
The two-cycle engine has been known as an engine that is compact but can produce a high output because of its relatively simple structure and mechanism. This engine has been widely applied to motorcycles, small boats, and so on.
In this two-cycle engine, valves are disposed near exhaust ports, and are opened or closed in order to control exhaust gases, thereby enabling the engine to operate under optimum conditions.
For instance, the two-cycle engine disclosed in Japanese Patent Laid-Open Publication No. Hei 7-71279 or No. Hei 7-180556 includes exhaust control valves disposed on inner walls of exhaust passages near upper edges of exhaust ports in order to open or close the exhaust ports. When the engine is operating at a low speed or under a low load, the exhaust control valves substantially fully close the exhaust ports, optimally controlling pressure in the cylinders, activating fresh air in combustion chambers using thermal energy of already burnt gases remaining therein, realizing combustion in an active and hot atmosphere in which self-ignition combustion is carried out, and improving fuel economy and purification of exhaust gases.
Further, Japanese Patent Laid-Open Publication No. Sho 56-56915 exemplifies the two-cycle engine in which exhaust control valves for varying exhaust emission timing are positioned on inner walls of exhaust passages near upper edges of exhaust ports. When the engine is operating at a high speed, the exhaust control valves are fully opened to advance the timing for opening the exhaust ports (i.e., exhaust emission timing), thereby promoting discharge of exhaust gases. Conversely, when the engine is operating at a low speed, the exhaust control valves are nearly closed to retard the exhaust timing end prevent fresh air from blowing through the cylinders.
In the foregoing two-cycle engines, the exhaust control valves are supported by a pivot, are positioned near the upper edges of the exhaust ports, and are activated to vary the open areas of the exhaust ports.
Each exhaust control valve has an edge (i.e., the edge for opening or closing each exhaust port) which is in contact with an inner surface of the exhaust port. This means that the pivot for supporting the exhaust control valves should be positioned near the exhaust ports. If the pivot is positioned far from the exhaust ports, the exhaust control valves are subject to an increase in their turning radius, and suffer from reduced rigidity in their moving parts and an enlarged structure.
Various proposals have been made in order to rotatably position the exhaust control valves near the exhaust ports. For instance, Japanese Utility Model Publication No. Sho 57-12176 discloses the two-cycle engine, in which a cylinder block where exhaust ports are present comprises main and auxiliary cylinder block parts, and a pivot is disposed between these cylinder block parts in order to rotatably support the exhaust control valves near the exhaust ports.
However, in the foregoing two-cycle engines, the exhaust control valves are directly supported by the cylinder block where the exhaust ports are formed. Therefore, the two-cycle engines have a problem that the cylinder block should be subject to a complicated machining process of forming bearings and so on for receiving the pivot, so that it is difficult to attach the exhaust control valves to the cylinder block. Further, when the pivot is interposed between the cylinder block parts as in the last mentioned publication, the bearings should be precisely formed on surfaces of the cylinder block parts to be joined. This means reduced efficiency in manufacturing engines.
Since the exhaust control valves are exposed to hot exhaust gases, the bearings and so on should have a cooling member. However, such a cooling member is difficult to add because a structure for mounting the exhaust control valves becomes complicated.
Japanese Patent Laid-Open Publication No. Sho 62-35013 discloses the jet-propelled boat including an exhaust system, in which a first exhaust pipe communicating with an exhaust outlet of an engine is connected to an exhaust gas expanding chamber at its one end, and the exhaust gas expanding chamber is connected to an exhaust gas discharging outlet using a second exhaust pipe. The other end of the first exhaust pipe projects to the center of the exhaust gas expanding chamber, and extends substantially vertically in the boat. In other words, this jet-propelled boat prevents water, which enters into the exhaust gas expanding chamber via the exhaust gas discharging outlet because of sudden acceleration or waves, from entering into the engine via the first exhaust pipe.
The jet-propelled boat disclosed in Japanese Utility Model Laid-Open Publication No. Hei 5-27270 includes a valve which is interposed in an air intake path between an air inlet for introducing fresh air and an air inlet of an engine in order to open or close the air intake path. Further, the valve is interlocked to a float positioned outside the air intake path. When outside water enters into the boat, the float is lifted by the water to let the valve close the air intake path, thereby preventing the water from entering into the engine.
Various measures have been taken in order to protect the jet-propelled boat against water entering thereinto, as described above. Further, there has been a great demand for improving the ease of driving of such a small jet-propelled boat.